In addition to the five senses humans and most other vertebrates experience, some fish have a sixth - the ability to detect electrical fields in their environment. These 'weakly electric' fish, principally the African Mormyriformes and the South American Gymnotidae, have developed a unique electrosensory system which consists of an organ that generates pulse-type electrical signals and an array of subcutaneous electroreceptor cells which detect electrical signals. Weakly electric fish use these unique adaptations to detect and identify objects and organisms in their environment and to communicate with other weakly electric fish.

Anatomy of the Electrosensory System

The electrosensory system consists of the electric organ which generates electric signals and the array of electroreceptor cells which detect electrical fields in the fish's environment.

The electric organ is located in the tail of a weakly electric fish. It consists of between two and five rows of electrocytes, modified muscle or nerve cells that produce the fish's electric organ discharge, or EOD. The EOD is quite small, typically less than one volt, which sets weakly electric fish apart from the better-known electric eel, ray, and catfish; these produce discharges as strong as 600 volts. There are several different forms of electrocytes which have been documented in different species, and both the type of electrocyte and the dimensions of the whole electric organ determine the waveform of the EOD.

The size and shape of the electric organ is apparently adapted to a given species' environment. Weakly electric fish native to highly conductive water tend to have short, broad electric organs, while those that live in less conductive water have long, thin organs; presumably these shapes generate EODs that transmit better in the fish's native waters (9).

The electroreceptor array is spread across the body of a weakly electric fish, just under the skin. The skin itself is not very electrically resistive, which presumably improves the sensitivity of the electroreceptor cells (5).

Just as with electrocytes, there are a variety of different types of electroreceptors, each sensitive to a different parameter of electric signals such as signal strength, polarity, and phase. Study of the banded knifefish, species Gymnotus carapo, has shown that electroreceptors are especially concentrated on the head and around the mouth, which is handy for detection of the electrical fields generated by prey animals (6).

Active Electrolocation

Active electrolocation is possibly the most interesting application of the electrosensory system. It allows weakly electric fish to detect and identify objects without seeing them at all, and it works with remarkable accuracy. Since they often live nocturnal lifestyles in murky river waters, weakly electric fish make good use of this ability.

The essential process is simple: a fish's electric organ produces a continuous stream of EODs, which sets up an electric field around the fish, making it rather like a magnet (11). The fish's electroreceptor array detects this field. Whenever an organism or obstacle moves within the field, it creates a distortion in it, and the electroreceptors detect the change in the field. The range of active electrolocation is about one body length, and very small objects or objects with resistance values similar to that of water are not detectable until quite close (16).

In fact, objects cast a sort of distortion shadow onto the electroreceptor array (ie, the fish's skin). The shadow is 'darkest' at the centre (where the distortion is greatest) and fades outward. It grows larger as the distance to the object increases, quite the opposite of light images, which grow smaller with increasing distance. It might appear that the fish could judge the distance between itself and an object based upon the width of the shadow, much as the eye judges distance based upon image size, but this is not the case, since a large object at close range might cast a shadow similar to a small object at long range. In fact weakly electric fish seem to measure distances using the ratio of the maximum distortion and the maximum slope of its profile; in simpler terms, it compares the darkness and the fuzziness of the shadow to determine range (17).

Weakly electric fish can also use the distortions created by objects within their electric fields to identify them. Tests have shown that such fish can discriminate between objects with different resistance and capacitance values, which comes in handy when it's necessary to decide whether to attack, run away from, or ignore an object that enters the fish's field (15).

Passive Electrolocation

Passive electrolocation operates much like active electrolocation, except that in passive electrolocation a weakly electric fish detects and analyses electrical fields generated by other organisms and objects rather than detecting changes in its own field (14). It's similar to the difference between echolocation and hearing.

Passive electrolocation presents rather different challenges from other senses, since it's based on detecting electromagnetic fields. Unlike sound waves, electric fields offer no velocity variables. (That is, electric fields offer no clue to the velocity of their source. Sound waves do because of the Doppler effect, in which sounds generated by sources approaching the listener seem to have higher pitches, and sounds generated by sources travelling away from the listener seem to have lower pitches.) So, since natural sources of electric fields tend to be dipolar, the field vectors do not point directly at their sources. Tests with Brachyhypopomus diazi and G. carapo have shown that the fish bend their bodies to track signals whose sources suddenly change location, and G. carapo failed to find an electrode that was switched off while the fish approached; apparently the specimens need continuous signals to track (13).

The paddlefish, Polydon spathula, has been shown to use passive electrolocation to detect its prey, the water flea Daphnia. In tests, paddlefish attacked electrodes that produced signals to mimic water fleas, but if they were not rewarded with food, they soon learned not to bother (18).

Communication

Weakly electric fish can, consciously or unconsciously, encode useful information in the waveforms of their EODs, which can then be detected and interpreted by other weakly electric fish. It has been mentioned previously that weakly electric fish of different species have differently shaped electric organs made up of different types of electrocytes, which produce species-specific EODs; there is also variability within species between the sexes and between individuals (9).

Within species, fish communicate to help establish dominance hierarchies and to attract mates. G. carapo apparently uses information in other fishes' EODs to decide how to respond to them, and even modulates its EOD waveform to signal submission to dominant individuals (3). Since increasing the range of the EOD is energetically expensive, female weakly electric fish probably use males' EOD ranges to decide whether or not they'd make good, healthy mates (9).

Some weakly electric fish incorporate electric communication into more complex mating displays. Males of the African species Polliomyrus isdori and P. adspersus attract females using sound-based mating calls, but they wait until they detect a 'feminine' EOD before starting (7).

Links for Further Study

• This page, by Masashi Kawasaki at the University of Virginia, has further information about all varieties of electric fish and some very helpful diagrams.
• The Journal of Experimental Biology's web site was a huge help in the research for this entry; it provides free .PDF downloads of recent articles on all sorts of interesting topics, including weakly electric fish.

Bibliography

Works specifically cited and researched

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